How many adult oysters are in the Great Bay Estuary and how has it changed over time?

The number of adult oysters decreased from over 25 million in 1993 to 1.2 million in 2000. Since 2012, the population has averaged 2.1 million oysters, which is 28% of the PREP goal for oyster recovery by 2020. This shows a decline from the previous reporting period (2009-2011) which averaged just over 2.8 million oysters.

Filter-feeding oysters are both a fisheries resource and a provider of key ecosystem services and functions. For example, they can reduce phytoplankton biomass and other suspended particles; this increases the ability for light to penetrate through the water which helps benthic plants, like eelgrass, to grow. They also provide important habitat for many invertebrate species and enhance biodiversity. Since the early 1990’s as oyster populations in the Great Bay Estuary have declined, it is likely these important functions and services that oysters provide, may have also declined.
Increase the abundance of adult oysters at the six documented beds in the Great Bay Estuary to 10 million oysters by 2020.

From 2012 to 2016, the average standing stock of adult oysters (greater than 80 mm in shell height) at the six largest oyster habitat sites (Figure 14.1) was just over 2.1 million oysters. This shows a decline from the previous reporting period (2009-2011) which averaged just over 2.8 million oysters (Figure 14.2). In 2016, there were 2,766,314 oysters, a decrease of 89% from 1993, when 25,729,204 adult oysters were present. The 2016 oyster population is approximately 28% of the PREP goal.

A primary limitation on oyster health is disease, caused by two microscopic parasitic organisms, Dermo (Parkinsus marinus) and MSX (Haplosporidium nelsoni). Figure 14.3 shows that Dermo, a warmer water organism, has become more prevalent over time. The prevalence of both diseases increase with salinity.52 Figure 14.3 also indicates that oysters no longer grow above 115 mm in shell height, which suggests that oysters are only living four or five years, rather than 10+ years as they did in the early 1990s.

Oyster habitat in the Great Bay Estuary also faces challenges due to available substrate for oyster larvae to settle. Oysters themselves can provide this substrate, but less and less oyster habitat diminishes the available substrate. This can be offset by planting recycled oyster shell material—for example, from restaurants and other sources—in key locations in the estuary (see Oyster Restoration).

Sedimentation is another stressor on oysters and it relates to the issue of available substrate. Sediments occur in the watershed from run-off, from stream and river erosion, and they get resuspended from the substrate in the estuary. With eelgrass and oyster habitats decreased from historic levels, sediments may be more easily resuspended following storms and high-flow periods. Oyster restoration monitoring has indicated that young reefs can easily be smothered by sediment.

Recreational harvesting of oysters may also be stressing the population. However, studies from other areas have shown that some restricted harvesting can provide benefit, through the removal of sediment.

Figure 14.1 Map showing the locations of the six major oyster beds in the Great Bay Estuary.

Figure 14.2 Standing stock of adult (>80 mm shell height) oysters in the Great Bay Estuary. Standing stock is estimated by multiplying adult densities by estimates of the acreage at each site.

Figure 14.3 Maximum shell height of oysters from the Adams Point, Nannie Island, and Woodman Point reefs. Updated from the original graph, published in Eckert (2016), available at https://scholars.unh.edu/prep/371.